The present invention relates to a method for producing an optical fiber by first producing a tubular glass body having concentric regions of different indices of refraction and drawing the glass body into a glass fiber.
Optically conductive glass fibers are drawn from preforms. According to the CVD [chemical vapor deposition] process, a tube comprising a glass jacket is coated with a glass core in its interior. The glass core is composed, in particular, of silicon dioxide (SiO.sub.2) doped with germanium dioxide (GeO.sub.2), and has a higher index of refraction than the glass jacket. With the OVD [outside vapor deposition] process, it is also possible to first coat a rod-shaped form with a doped glass core and then coat the glass core with a glass jacket, and ultimately removing the mandrell. In both cases, a tubular glass body is obtained, in some cases after vitrifying the applied layers. The tubular glass body is then heated, beginning at one end and progressing along its length, until the softening point is reached and the softened tube collapses into a solid body. The collapsing may be a separate process step or may be effected together with drawing the fiber.
In the glass core, particularly when doped with GeO.sub.2, an annoying dip in the index of refraction has been observed in its central region. This dip is the result of the high heat required in the collapsing and fiber drawing steps, which causes GeO.sub.2 to be evaporated, primarily from the tube's interior surface. Consequently, a central region poorer in GeO.sub.2 is produced in the collapsed core. Such a dip in refractive index, originally produced in the preform, is then also present, in a geometrically equivalent shape, in the drawn optical fiber.
This undesirable dip in the index of refraction, depending on its size, produces the following disadvantageous physical effects in:
Multimode graded Index fibers: PA0 Monomode fibers:
reduction of coupling efficiency; PA1 reduction of transmission bandwidth; PA1 error interpretation in the DMD [Differential Mode Delay] measuring method; PA1 negative influence on the concatinations factor in large optical fiber path lengths; and PA1 sensitivity of bandwidth measurement with respect to launching conditions and microbending effects PA1 increase of microcurvature sensitivity PA1 influence on cut off wavelength
In spite of numerous experiments, it has not been possible in the past to completely avoid the refractive index dip; it was merely possible to reduce it. One prior art method achieved a small reduction of the refractive index dip by precollapsing the initially larger interior diameter of the coated tube to form a hollow center space of lesser diameter. After this reduction in size, the GeO.sub.2 deficient inner surface layer was removed by etching. This precollapsing and etching process was performed until the smallest possible inner diameter remained, such that during the subsequent final collapsing step only a relatively small percentage of GeO.sub.2 deficient area remained to produce a small dip zone. Another known method for reducing the dip is to add germanium containing substances during the collapsing step to counteract the GeO.sub.2 reduction.